The invention relates to tools for forming internal threads on either generally smooth-sided bores through plate members or on previously threaded members that are to be re-shaped and, more particularly, relates to such a tool for cutting, rolling or otherwise reshaping threads in a bore that may be either of an unusually large diameter or of a non-standard diameter for which conventional thread taps either do not exist or are difficult or expensive to obtain. A tool constructed according to the invention can also be used to advantage in forming threads of a standard thread-tap size where the risk of damaging an expensive standard size thread tap is great enough to recommend use of the less expensive tap construction of the invention.
The use of thread tap tools to form internal screw threads on bores or other passageways through nuts or metal plate members is generally well-known. A large number of standardized sizes of different diameter thread taps are generally readily available in commerce for that purpose. Thus, it is often possible to readily obtain a standard thread tap for forming threads of a given desired size or pitch on the internal walls of a bore of a desired size. However, as the size of standard thread taps increases, the cost and often the unavailability, of such taps increases proportionately. Accordingly, for those wishing to form threads on a relatively large diameter bore, and having only infrequent need to form such threads, the cost of obtaining such a large-diameter thread tap may be prohibitive.
In addition to that problem associated with using commercially available prior art thread taps, there also exists the further difficulty of readily obtaining a thread tap having a particular diameter or thread pitch that may be desired for forming threads at a given time or location. For example, often in the repair or maintenance of machinery at locations where a large variety of commercial thread taps are not readily available, it is necessary to re-cut threads in an existing large diameter or non-standard bore on the machinery in order to expeditiously complete the repair or maintenance work. Before the present invention, the only solution to such problems was to incur the time penalty and expense associated with finding and acquiring a standard thread tap having the particular diameter and thread pitch needed to match the existing damaged threads. Of course, a less expensive and less time-consuming alternative would be desirable if one could be found.
A further disadvantage of prior art thread taps of large diameters is that they are expensive and can be damaged or rapidly worn out, particularly when used to cut threads in hardened metals. Thus, it would be desirable to provide a less expensive means for forming large-diameter threads particularly in hardened metals.
It would also be desirable to provide a conveniently usable thread tap that can be readily adjusted to form threads of different sizes in a plate member, thereby to obviate the earlier requirement of using more than a single standard-size tap to achieve such an objective.